The present invention relates to semiconductor devices and methods of operating the same, and more particularly, to nonvolatile memory devices and methods of operating the same.
In recent years, due to the increased demands for semiconductor devices applicable to various fields, a vast amount of research has been conducted on memory devices that are capable of meeting various requirements, such as large capacity, small size, high speed, low power consumption, and high integration density.
For example, ferroelectric random access memory devices (FeRAMs), magnetic RAMs (MRAMs), and ovonic unified RAMs (OUMs) have been proposed as advanced nonvolatile memory devices. An FRAM, which uses spontaneous polarization of ferroelectric materials, has potential advantages of low power consumption and high-speed operation. However, the FRAM can have high fabrication costs and can have relatively poor data retention characteristics. An MRAM is a ferromagnetic tunneling device using a giant magneto-resistive (GMR) effect. The MRAM can consume relatively high power for magnetization switching and can have certain technical limitations on increasing the degree of integration. A phase-change RAM (PRAM), such as an OUM, can consume a relatively high amount of power for switching a current.
In order to address some issues of the above-described RAMs, a resistive RAM (RRAM) using an electric pulse induced resistive (EPIR) effect has been proposed. The RRAM can reduce power consumption and/or increase the degree of integration, and its resistance can vary within a relatively wide range, which may permit realization of multi-bit storages.
A conventional EPIR device uses a variable resistor having a Perovskite structure based on a network of oxygen-octahedrals centering on 3d-transition metallic elements, more specifically, Pr1−xCaxMnO3 (hereinafter referred to as a PCMO), La1−xCaxMnO3, La1−xSrxMnO3, Gd0.7Ca0.3BaCO2O5+5 and the like. It has been said that of these materials, PCMO having a composition close to x=0.3 may show a wide range of resistance value variation. However, forming a PCMO layer having a uniform Perovskite structure is difficult, and fabrication of the variable resistor should not be followed by a process performed at a high temperature (e.g. 400° C. or higher) because one or more properties of the variable resistor may be changed by the high temperature. Further, it is known that a decrease of resistance can require a pulse of 1 to 100 μs and a voltage of 0.5 to 10 V, and an increase of resistance can require a pulse of 10 to 1000 ns and a voltage of about 1.5 to 2.5 times the voltage required by the decrease of resistance.